1. Field of the Invention
The invention relates to permeable membranes for the separation of gases. More particularly, it relates to permeable membranes having enhanced gas separation/permeability characteristics.
2. Description of the Prior Art
Permeable membranes capable of selectively permeating one component of a gas mixture are considered in the art as convenient, potentially highly advantageous means for achieving desirable gas separations. For practical commercial operations, permeable membranes must be capable of achieving an acceptable level of selectivity of separation of the gases in a feed stream while, at the same time, achieving a desirably high productivity of gas separation.
Various types of permeable membranes have been proposed in the art for the carrying out of a variety of gas separation operations. Such membranes can generally be classified as being of the (1) isotropic, (2) asymmetric or (3) composite type. The so-called isotropic and asymmetric type membranes are comprised essentially of a single permeable membrane material capable of selectively separating desired components of a gas mixture. Isotropic membranes have the same density throughout the thickness thereof. Such membranes generally have the disadvantage of low permeability, i.e. low permeate flux, due to the relatively high membrane thickness necessarily associated therewith. Asymmetric membranes are distinguished by the existence of two distinct morphological regions within the membrane structure. One such region comprises a thin, dense semipermeable skin capable of selectively permeating one component of a gas mixture. The other region comprises a less dense, porous, non-selective support region that serves to preclude the collapse of the thin skin region of the membrane under pressure.
Composite membranes generally comprise a thin layer or coating of a suitable permeable membrane material superimposed on a porous substrate. The separation layer, which determines the separation characteristics of the composite structure, is advantageously very thin so as to provide the desirably high permeability referred to above. The substrate only serves to provide a support for the thin membrane layer positioned thereon.
As the advantages of permeable membranes have become increasingly appreciated in the art, the performance requirements of such membranes have likewise increased. Thus, the art is moving in the direction of very thin membranes having desirable permeability characteristics without sacrifice of the separation, or selectivity, characteristics of the hollow fiber or other permeable membrane structure. It is thus increasingly desired that more advantageous combinations of permeability and selectivity be achieved with respect to a variety of as separations of commercial interest. As indicated above, isotropic type membranes are not generally suitable for the achieving of such requirements. Asymmetric membranes, on the other hand, can be developed for such practical gas separation applications, but do not possess an inherent flexibility enabling them to be readily optimized for particular gas separation applications. While the thin dense, semipermeable layer of a particular asymmetric membrane material can be made thinner for increased permeability, the selectivity characteristics of said material, unless modified by particular treatment techniques, may be no more than adequate with respect to the separation of the components of the gas being treated in the particular application.
The thin skin of conventional asymmetric membranes, such as are described in the Loeb U.S. Pat. No. 3,133,132, is generally found not to be perfect, but to contain various defects. Such defects, in the form of residual pores, minute pinholes and the like, comprise relatively large size openings through which the feed gas passed to a membrane of such material will preferentially flow. As a result, a significantly reduced amount of gas separation due to the interaction of the feed gas with the material of the permeation membrane itself will occur as a result of the presence of such defects in the membrane structure. In the case of asymmetric polysulfone hollow fibers, such defects result in the selectivity for O.sub.2 /N.sub.2 separation being only in the range of about 1-1.5 as contrasted to a selectivity for O.sub.2 /N.sub.2 of about 6.0 for polysulfone that is free of defects. As used herein, it will be understood that the selectivity, or separation factor, of a membrane or membrane module assembly, represents the ratio of the permeate rate of the more readily permeable component to the less readily permeable component of a particular gas mixture. In a proposed solution to this problem, Henis et al., U.S. Pat. No. 4,230,463, disclosed the coating of the asymmetric membrane with a coating material having a determined intrinsic separation factor that is less than that of the material of the separation membrane. The resulting multicomponent membrane was found to exhibit a separation factor significantly greater than the determined intrinsic separation factor of the material of the coating and greater than the separation factor exhibited by the uncoated separation membrane. Using this approach, silicone, having a selectivity for O.sub.2 /N.sub.2 separation of about 2, can be coated on polysulfone hollow fibers to increase the O.sub.2 /N.sub.2 selectivity thereof from the 1-1.5 range indicated above to from 2 to 6, with such O.sub.2 /N.sub.2 selectivity commonly approaching 6. The permeability (as defined below) of such silicone/polysulfone composites have generally been relatively low, i.e. about 0.2 ft.sup.3 (STP)/ft.sup.2. day psi. or less, leading to the desire for thinner membranes, i.e. thinner dense skins, particularly in light of the increasing requirements in the art for high flux operation. Thinner membranes lead, however, to an increase in the number of defects that require curing to achieve acceptable performance. While efforts to improve this approach continue, there remains a desire in the art for the other approaches to provide a desirable combination of selectivity and permeability for practical commercial operation. For such reasons, composite membranes, utilizing membrane materials selected particularly for a desired gas separation, offer the greatest opportunity, with respect to particular gas separations of commercial interest, for the achieving of desirable combinations of selectivity and permeability. It will be appreciated that composite membranes, to achieve the performance requirements desired in the art, must not only incorporate very thin membrane layers, but must comprise separation layer-substrate structures of optimum advantage for a desired gas separation operation. Illustrative examples of the wide variety of practical commercial operations in which composite membranes may be advantageously employed include air separation, the recovery of hydrogen from ammonia purge gas and from refinery streams, carbon dioxide and methane separations on a variety of operations, helium and methane separations, and the like.
There is a genuine need and desire in the art, therefore, to develop unique composite type membranes capable of providing enhanced gas separation in practical commercial operations. It is also desired in the art that the processes for producing such composite membranes be improved so as to enhance the effectiveness and reliability of such membranes.
It is an object of the invention, therefore, to provide permeable membranes having enhanced gas separation characteristics.
It is another object of the invention to provide composite membranes having advantageous combinations of selectivity and permeability for desired gas separations.
It is a further object of the invention to provide a process for the preparation of improved permeable membranes having enhanced effectiveness and reliability.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.